Move path info caching from BinaryCacheStore to Store

Caching path info is generally useful. For instance, it speeds up "nix
path-info -rS /run/current-system" (i.e. showing the closure sizes of
all paths in the closure of the current system) from 5.6s to 0.15s.

This also eliminates some APIs like Store::queryDeriver() and
Store::queryReferences().

src/libutil/ref.hh

@@ -43,6 +43,12 @@ public:
         return p;
     }
 
+    template<typename T2>
+    ref<T2> cast()
+    {
+        return ref<T2>(std::dynamic_pointer_cast<T2>(p));
+    }
+
     template<typename T2>
     operator ref<T2> ()
     {